Ground and surface waters in urban areas are susceptible to nitrate contamination from septic systems, leaking sewer lines, and fertilizer applications. Source identi®cation is a primary step toward a successful remediation plan in aected areas. In this respect, nitrogen and oxygen isotope ratios of nitrate, in conjunction with hydrologic data and water chemistry, have proven valuable in urban studies from Austin, Texas, and Tacoma, Washington.In Austin, stream water was sampled during strem¯ow and base¯ow conditions to assess surface and subsurface sources of nitrate, respectively. In Tacoma, well waters were sampled in adjacent sewered and un-sewered areas to determine if locally high nitrate concentrations were caused by septic systems in the un-sewered areas. In both studies, sewage was identi®ed as a nitrate source and mixing between sewage and other sources of nitrate was apparent. In addition to source identi®cation, combined nitrogen and oxygen isotopes were important in determining the signi®cance of denitri®cation, which can complicate source assessment by reducing nitrate concentrations and increasing d15 N values. The two studies illustrate the value of nitrogen and oxygen isotopes of nitrate for forensic applications in urban areas.# Published by Elsevier Science Ltd. on behalf of AEHS.

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Each year, approximately 11,000 tons of inorganic nitrogen and 2,100 tons of total phosphorus are transported by rivers and streams to Puget Sound and its adjacent waters Rivers that have the largest streamflows carry the largest nutrient loads Nutrient yields are largest from basins with higher percentages of urban and agricultural areas and that receive the highest inputs of nutrients Why are nutrients important in the Puget Sound Basin?

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FOREWORDThe mission of the U.S. Geological Survey (USGS) is to assess the quantity and quality of the earth resources of the Nation and to provide information that will assist resource managers and policymakers at Federal, State, and local levels in making sound decisions. Assessment of water-quality conditions and trends is an important part of this overall mission.One of the greatest challenges faced by waterresources scientists is acquiring reliable information that will guide the use and protection of the Nation's water resources. That challenge is being addressed by Federal, State, interstate, and local water-resource agencies and by many academic institutions. These organizations are collecting water-quality data for a host of purposes that include: compliance with permits and water-supply standards; development of remediation plans for a specific contamination problem; operational decisions on industrial, wastewater, or water-supply facilities; and research on factors that affect water quality. An additional need for water-quality information is to provide a basis on which regional and national-level policy decisions can be based. Wise decisions must be based on sound information. As a society we need to know whether certain types of water-quality problems are isolated or ubiquitous, whether there are significant differences in conditions among regions, whether the conditions are changing over time, and why these conditions change from place to place and over time. The information can be used to help determine the efficacy of existing water-quality policies and to help analysts determine the need for and likely consequences of new policies.To address these needs, the Congress appropriated funds in 1986 for the USGS to begin a pilot program in seven project areas to develop and refine the National Water-Quality Assessment (NAWQA) Program. In 1991, the USGS began full implementation of the program. The NAWQA Program builds upon an existing base of water-quality studies of the USGS, as well as those of other Federal, State, and local agencies. The objectives of the NAWQA Program are to:Describe current water-quality conditions for a large part of the Nation's freshwater streams, rivers, and aquifers.Describe how water quality is changing over time.Improve understanding of the primary natural and human factors that affect water-quality conditions. This information will help support the development and evaluation of management, regulatory, and monitoring decisions by other Federal, State, and local agencies to protect, use, and enhance water resources.The goals of the NAWQA Program are being achieved through ongoing and proposed investigations of 60 of the Nation's most important river basins and aquifer systems, which are referred to as study units. These study units are distributed throughout the Nation and cover a diversity of hydrogeologic settings. More than two-thirds of the Nation's freshwater use occurs within the 60 study units and more than two-thirds of the people served by public water-supply syste...